This invention relates generally to resistance heaters, and more particularly to thermostatically controlled resistance heaters for use in the sump well of a compressor.
Prior types of resistance heaters have generally included a thermally conductive outer sheath having one or more compartments within which is located an electrical resistance heating element and a thermostat for controlling the energization of the heating element. The heating element is spaced away from the sides of the sheath and a thermally conductive but electrically insulative fill material is located in the space between the sheath and the heating element.
In these prior types of heaters, the thermostat is mounted within the sheath such that heat conduction takes place from the heater to the thermostat along a path which is enclosed within but does not pass through the sheath. A consequence of this type of arrangement is that the thermostat is exposed to higher temperatures than the object or material to be heated, and hence the thermostat may de-energize the heater before the material to be heated reaches the desired temperature.
Moreover, the spacing of the heating element from the outer sheath decreases the heating efficiency of the heater due to the less than perfect thermal conductivity of the intervening fill material.
Other types of resistance heaters utilize circular cylindrical cores having a plurality of channels through which a resistance heater is wound. The channels, however, are not located at equal distances from the outer sheath, and hence the heat developed by the separate portions or legs of the resistance heater within the channels tends to accumulate within the core due to the differing path lengths of heat conduction from the legs. It has been found that this build-up of heat causes premature switching of the thermostat, resulting in short cycling of the resistance heater.